1

Paper ID: 149

Increasing efficiency of the conventional

auxiliary systems of power plants (Reduction

of Life Cycle Cost by operational excellence)

Thomas Schmager Pasi Mannistö Per Wikström

ABB Switzerland Ltd ABB Switzerland Ltd ABB Switzerland Ltd

Switzerland Switzerland Switzerland

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Contents

1. Abstract .................................................................................................................................3 2. Introduction...........................................................................................................................3 3. Basics of fans ........................................................................................................................6

3.1. Controlling the flow of fans ....................................................................................... 6 3.2. Constant speed fan ..................................................................................................... 8 3.3. Variable speed fan.................................................................................................... 11

4. Design and operation of large (variable speed) fans in power plants .................................11 5. Electrical operating conditions in power plants..................................................................13

5.1. Comparison of operational conditions ..................................................................... 13 6. Economical benefits calculation for electrical drive systems .............................................13 7. Advantages of variable speed fans driven by medium voltage AC drives .........................18

7.1. Energy efficiency ..................................................................................................... 18 7.2. Reliability, controllability, operability..................................................................... 18 7.3. Soft starting capability ............................................................................................. 20 7.4. Reduced maintenance .............................................................................................. 20 7.5. Reduced investment in other parts of plant.............................................................. 20

8. Medium voltage AC drives .................................................................................................21 9. Summary .............................................................................................................................22

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1. Abstract

Booming demand for electricity, especially in the developing countries, has raised power generation

technologies in the headlines. At the same time the discussion about causes of global warming has

focused on emissions originating from power generation and on CO2 reduction technologies such

as:

alternative primary energy sources,

capture and storage of CO2,

increasing the efficiency of converting primary energy content into electricity.

Concerning the third focus point, relatively little attention has been paid to the efficiency of

auxiliary processes in thermal power plants, which account for about 5-8% of the gross generation

capacity. Examples of auxiliary processes in thermal power plants:

conveying and preparing the fuel

moving the necessary air into the furnace

moving the flue gases from the furnace

returning the condensed water back to the steam generator

maintaining the necessary cooling effect in the condenser and

operating various emission cleaning processes.

A majority of these processes are run by centrifugal pumps, fans and compressors driven by large

electrical motors.

This paper describes existing and proven possibilities to successfully control such processes while

at the same time reducing the auxiliary power consumption in power plants by using modern

electric variable speed drives. A detailed technical and economical analysis of an Induced Draft

(ID) fan is provided. The paper describes how small investments can provide attractive methods to

increase the net generation capacity and at the same time reduce emissions and fuel consumption.

2. Introduction

Rapid growth of electrical energy demand, not only in developing countries, discussions on fossil

fuel reserves and the impact of thermal power generation on global warming, have increased the

focus on alternative primary energy sources and the efficiency improvement techniques for the

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conversion of the fossil fuels into electricity. Development efforts are ongoing to reduce, capture

and/or store CO2 emitted from burning fossil fuels.

The following is a quote from the McKinsey report of May 2007 “Curbing the energy demand

growth”.

QUOTE

Reducing current losses from electricity generation and distribution is another substantial

opportunity. Power generation used 155 QBTUs (Quad =1015) – representing a hefty 37% of global

energy use – to generate 57 QBTUs of deliverable electricity in 2003. In short, close to two-thirds

of the energy put to the process is lost before it reaches the final end user.

UNQUOTE

In this essay, we will look at some of the auxiliary load in fossil-fueled power stations and see what

can be done to reduce this part of the losses.

Thermal power stations use 3 to 8 % of their gross generation capacity for auxiliary processes. A

conventional coal-fired thermal power plant uses slightly more (5 – 8%) of the electricity it

produces for the auxiliary load. For a combined-cycle power plant, the auxiliary consumption can

be less than 3.5 %.

Auxiliary processes are required to keep the generator running; they are, for instance, conveying

fuel coal to coal mills and maintaining the cooling water flowing through the condenser. Most of

the auxiliary power demand, up to 80%, is used by large electric motors that are typically connected

to the medium voltage switchboard, supplied through auxiliary transformers. The increasing

demand for “clean coal” and CO2 emission capture and storage technologies will increase the

auxiliary electricity consumption of electric power generation.

Power stations, especially base-load ones, are running at full load all the time. It is easy to imagine

that not much change or control is needed for the auxiliary processes in such cases. The fact is

however, that very few power stations run at their maximum capacity throughout the year; instead

the capacity is being adjusted all the time to match demand and various operation conditions and

parameters. See Figure 1 illustrating a one year capacity utilization curve and Figure 2 for the

corresponding operating hours of a 660 MW steam generator.

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Generator output

0%

20%

40%

60%

80%

100%

Annual hours [%]

Figure 1: Capacity utilization curve of a 660 MW steam generator.

Figure 2: Operating hours at various load ranges of a 660 MW steam generator.

Auxiliary load machines, like centrifugal blowers, pumps or compressors, must be designed to

operate reliably under all operating conditions, which leads to part load operation most of the time.

This paper focuses on efficiency improvements of the plant’s own electricity consumption. Variable

1281

346

572

1838

2166

1458

1099

0-40

40-55

55-72

72-85

85-92

92-97

97 or greater

Sum of operating hours with generator % output of

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speed control technology is being introduced as alternative to traditional mechanical control

methods. The considerable economical benefits will be demonstrated in a case example.

The paper concludes that Variable Frequency Drives (VFDs) increase plant availability and

flexibility through improved process control. VFDs reduce maintenance costs and improve heat rate

by increasing the efficiency of auxiliary processes, which, in turn, reduces emissions. The improved

heat rate and power output results in a higher profitability and faster return on investment.

3. Basics of fans

Fans are devices used to produce a gas flow. Centrifugal fan behaviour is characterized by the

affinity laws, which are:

- flow is linearly proportional to the fan speed (if no counter-pressure)

- pressure changes with square of the ratio of speeds

- power demand (product of pressure and flow) changes with cubic of the ratio of speeds

3.1. Controlling the flow of fans

The gas flow is ducted to the process. The ductwork causes a pressure drop and defines the fan

operating point, where the pressure generated by the fan and the pressure drop caused by the

ducting and process are balanced (see Figure 3). Again, a change in the required gas pressure is

proportional to the change in flow squared. Thus, the system effect is illustrated by a quadratic

curve starting from the origin and crossing the head-capacity curve of the fan at the operating point.

When the system pressure drop increases or decreases, the inclination of the system curve increases

or decreases correspondingly.

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Pin

Atmosphere+

-

Absolute zero

Pout

Pta1

Pta2

Figure 3: Pressure curve in a fan system.

The electrical power demand also depends on the following:

- efficiency of the fan,

- efficiency of the electrical motor.

It must be noted that the efficiency of the fan depends on the selected flow control technology, and

some methods are clearly superior to others in this respect, as will be explained later in the paper.

The fan characteristic is illustrated with a diagram, head-capacity curve, showing the generated

pressure as a function of flow; see the examples in Figure 4 for a fixed speed fan and in Figure 5 for

a variable speed fan. From the curve it can be seen that a fan generates highest pressure at relatively

low flow rate and that the generated pressure is reduced when the flow increases. The stable

operation area is to the right of the maximum pressure point, the surge point. The equivalent

efficiency curves are also indicated in the figures. It is obvious that the fan efficiency drops

dramatically with reduced flow in Fig. 4, whereas it remains almost constant in Fig. 5.

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Figure 4: Fixed-speed fan graphs Figure 5: Variable speed fan graphs

The most commonly used fan types and the respective flow control methods are listed below and

described in the following sections.

Constant speed fan

• Damper control (outlet or inlet) • Guide vane control • Fan blade pitch control (for axial flow fans)

Variable speed fan

• Mechanical drive system • Electrical drive system

Figure 7 shows the comparison of the power demand versus volume flow for these control methods.

3.2. Constant speed fan

A fixed speed fan reaches its optimum efficiency at a certain pressure – flow point. The nominal

operation condition of a fan, called Test Block (TB), is typically designed to be very near to such

optimum operation point (ref. point “1” in Figure 4).

Each pair of two equal efficiency points on the fan curve are connected by an “isoefficiency” curve,

an elliptic curve formed by all the points that have the same efficiency. In an ideal world the fan

operating point would be located exactly at the efficiency optimum. In reality, the gas flow needs to

be controlled, however, it leads to a complex situation with regard to optimization of the fan

efficiency and the overall energy consumption. The gas flow control method has a big impact on the

energy consumption. The other lines in Figure 4 (marked with “-20”, “-40”, “-60” etc.) refer to the

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closing position of the applied inlet damper control. The inlet damper control does not change the

system curve, but moves the fan curve by reducing the available flow.

It must be noted from Figure 4 that the efficiency of a fixed speed centrifugal fan is always reduced

when flow control is applied, i.e. the operation point is moved away from the point of the optimum

efficiency. The gas flow within steam generators in power plants does always vary and the fans

need to be operated away from the optimum efficiency point due to the following reasons:

- the fans are initially designed with a certain overcapacity

- the operating conditions vary

- the loading conditions constantly vary

Outlet damper control (see Figure 6 for details)

With this method, the airflow is regulated by means of an outlet damper or other system resistance.

The fan is operated continuously at constant speed and the damper setting determines the flow. This

is the least efficient means of control because the system resistance is increased to reduce the flow.

In other words, the point of operation moves up on the fan curve as the dampers are closed.

Figure 6: Physical arrangement of various flow control devices for a fixed speed centrifugal fan

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Inlet damper control (see Figure 6 for details)

Flow control is achieved when the fan with inlet dampers is operated at constant speed, by varying

the position of parallel dampers. There are some power savings associated with the partial closure

because of the pre-rotation of the incoming air stream when the damper opens in the same direction

as the fan rotation.

Variable inlet vane control (see Figure 6 for details)

The fan operates at constant speed and with variable inlet vanes. Fan flow is controlled by varying

the pitch of the adjustable inlet vanes at the fan inlet. Fans with inlet vane controls, when operated

with partially closed vanes, provide reduced power consumption due to the pre-rotation of the

entering air or gas stream, when the inlet vanes open in the same direction as the fan rotation. The

power consumption of the fan with inlet vane control is higher than that with variable speed control

at reduced flow rates due to the throttling action as shown in Figure 7.

Fan blade pitch control

The angle of attack of the fan blades can be adjusted to vary the flow of axial fans. Blade pitch can

be adjusted either on the fly or manually when the fan is stopped. Fans equipped with pitch

adjustment on the fly are efficient within a certain range, but are characterized by relatively high

maintenance costs. This control method is only applicable to axial fans.

Figure 7: Power demand for different methods of flow control for radial fans

0

20

40

60

80

100

120

30 40 50 60 70 80 90 100% Design Flow

% D

esig

n kW

OUTLET DAMPER

INLET BOX DAMPER

VARIABLE INLET VANE

VARIABLEFREQUENCY DRIVE

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3.3. Variable speed fan

A variable speed fan has “zooming” capability; the head capacity curve moves up or down

maintaining its shape when the speed is changed. Again, the efficiency reaches its optimum at a

certain pressure – flow point for each speed, similar to a fixed speed fan, but since the speed can be

varied, the best efficiency point can also be moved. As the change in the required gas pressure is

proportional to the change in flow squared, each pair of two equal efficiency points on a variable

speed fan curve will form quadratic-shaped curves approximately following the shape of the system

curve (Figure 5).

This means that a variable speed fan can continue operation close to its maximum efficiency over

the entire control range. As can be seen from Figure 7, control of fan speed provides the highest

efficiency fan system. Medium voltage drives are the most efficient devices for controlling the

speed of large fan motors.

For instance, the two operating points in the variable speed fan curve in Figure 5 have the following

characteristics according to the fan data sheet: Please refer to Table 1. The reduction in the power

consumption between TB (Test Block) and MCR (Maximum Continuous Rating) is noticeable.

Table 1: Comparison of the operating points TB and MCR of a variable speed fan.

Point designation “1” “2”

Loading Design (TB) MCR

Efficiency [%] 88.8 87.2

Power at shaft [kW (%)] 3455 (100) 1856 (54)

Rotating speed [%] 100 80

4. Design and operation of large (variable speed) fans in power plants

An ID fan is used to suck away flue gases from the furnace of steam generators and to push them

further through the stack into the atmosphere. The gas flow needs to be controlled in order to

maintain ideal burning conditions inside the boiler. The ID fan also needs to generate a high enough

pressure for the gas stream, in order to overcome the counter pressures of air pre-heater,

electrostatic precipitator, FGD (Flue Gas Desulphurization) plant and the stack. Please refer to the

principle in Figure 3.

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Due to environmental regulations, a growing amount of FGD plants is being installed in power

stations with a remaining life time beyond 2012-2016. The addition of the FGD system usually

means that the existing ID fan output must increase to handle the additional pressure drop of the

FGD system. This will often necessitate a modification or a replacement of the ID fan. Many FGD

applications require drive motors up to or over 10,000 kW and hence an efficient system to start,

drive and control the fan is required.

The following are examples of varying operating conditions:

- fuel quality and moisture (e.g. the recently introduced popular bio-fuelled or RDF (Refuse Derived

Fuel) boilers

- ambient temperature

- function of environmental control process (DE-NOx, DE-SOx, precipitator)

- leakage, blockade

Most of the time fans are oversized to meet the TB condition. This is also true when the fan is

selected for a FGD application, where there are TB and MCR conditions. The selection of margins

between TB and MCR is usually done with pressure varying as the square of the flow rate.

For an illustration to how systems get oversized, please refer to ABB Review 2/2007 page 74

When fans are operated with conventional inlet damper and inlet vane control, the static efficiency

of the fan at the MCR condition is typically in the range of 70 to 80% and some times even lower

depending on the margin between the TB and MCR rating. This is approximately 10 to 20

percentage points below the peak efficiency of the fan, however, IEEE 958 mentions ratio between

MCR and TB being as low as 60 to 75%. If a variable speed drive is used for control, an ID fan can

be selected to operate at or close to the peak efficiency at TB and MCR. Please refer to Table 1.

Even base loaded units will typically show significant power savings for fans driven by variable

speed drives due to the initial over dimensioning.

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5. Electrical operating conditions in power plants

A power station is a challenging operation environment for electrical equipment. It is characterized

by high short-circuit capability, fluctuating voltage, supply interruptions due to high-speed bus

transfers, and very high availability requirements despite the prevailing conditions.

The operating conditions in power stations concerning installations of electric variable speed drives

can be found in many standards. The following gives a short overview of the main requirements in

selected countries:

5.1. Comparison of operational conditions

Normally, standards define conditions like supply voltage levels and variation, both long-term and

short-term dips and even interruptions.

Table 2: Comparison of basic electrical operating environment in power plants, various sources

Phenomena unit KEMA S-17 part

4 (Sep) Oct 1988

A Danish

project

SDRC (Chinese)

Industry[2004] 872 -

draft

Full output under supply

voltage variation

% +10/-15 +/-10 +/- 15

Operation under over-

/under-voltage

% +10/-30 +30 /-20

Interruption ms “few seconds” 300 3000

Frequency variations % - 5 +/-2 +/- 5

Today’s electric variable speed drives are or can specifically be designed to perform during the

strictest requirements mentioned in above table. Most electric variable speed drives can perform

either a power-loss-ride-through over short supply voltage interruptions up to a few seconds, or a

flying start after the voltage has resumed.

6. Economical benefits calculation for electrical drive systems

As shown in Figure 7, the most efficient way to control the speed of a fan is by using an electric

variable speed drive system. With the power consumption varying as the cube of the speed,

considerable savings in power consumption are achieved when the fan is operated at lower speeds.

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Large Induced Draft (ID) fans are perfect candidates for medium voltage AC drive technology,

where the energy savings can pay back the cost of the variable speed drives in a few years, or can

be used to help finance the overall project.

From a power stations’ total efficiency point of view the control technology of the auxiliary loads is

an important factor. The control method chosen (poor vs state-of-the-art technology) can make a

difference of up to 60% or more of the fan efficiency; as has been noticed in process industry [4].

From Figure 2 can be seen that even the loading of a large steam generator varies considerably. An

economically interesting question is: “How efficiently does the existing, or planned control

technology work.” To make an economical analysis on it, the key task is to calculate how much

power is needed to move the gas, of which the process values (temperature, density, pressure

variations and flow) are known. With the gas-moving power, the fan characteristic curve, the flow,

the pressure and efficiency, it is possible to calculate the mechanical power at the fan shaft over the

operating range. If detailed fan data or curves are not available, the actual energy consumption of

the driver motor could be taken out from the trend library of a DCS (Distributed Control System).

Models for economical evaluation are available in literature; for instance the sample in [1] suggests

the following three bands represent the load projections of a fan:

a) 25% of the time the fan is off line

b) 25% of the time the fan operates at 50% of the fan test block (TB)

c) 50% of the time the fan operates at 75% of TB

The steam generator annual loading, illustrated in Figure 1, is used in this example. The annual

hours were grouped into the indicated load projections in Figure 2.

In order to apply a general calculation method for a case where the exact fan curve is not available,

the fan curve in Figure 4 could be used for the steam generator, by unifying the flow and pressure

axes. It is assumed that the flow control methods are the same.

Key in analyzing the power consumption of a fixed speed fan with the flow controlled mechanically

by an inlet damper, and a variable speed fan, is to calculate the power consumption in each of the

selected operating points. In this case the flow at the TB is 610 m3/s and the pressure 656 mmca

(mm water column). Using a gas density of 0.829 kg/m3 and the fan peak efficiency of 85.8%

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indicated on the graph, we get PTB = 3’791 kW. The steam generator in question has two identical

ID fans in parallel, so the total power consumption per steam generator is twice this amount.

The power consumption at TB does not really help since the required gas flow through steam

generator is remarkably less than at TB, as was previously mentioned. If we assume the flow at

100% generator output being 90% of the TB, which may not even fulfill some of the most

conservative design margins, we can calculate the corresponding flow for each of the selected

generator load points. With these flow values, and the pressures and efficiencies read from the

normalized fan curve, the powers can be drawn for the fixed speed fan (damper control), see Figure

8.

0

500

1'000

1'500

2'000

2'500

3'000

3'500

45% 55% 65% 75% 85% 95% 105%Load

kW

Power (damper), kW Power (VFD), kW

Figure 8: Power consumption curves of fixed-speed (damper control) and speed controlled (VFD) ID fan

The losses (in % of the actual load) of a VFD system increase only marginally at part load. The

powers for the compared variable speed fan are simply calculated from the cubic power demand of

a fan taking into account the typical 3-5% losses of a VFD system at nominal load. The values can

easily be handled in a spreadsheet calculation, which for instance could look like in Table 3 below.

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Table 3: Example calculation spreadsheet

Load point 48% 64% 79% 89% 95% 99% TBHours 346 572 1838 2166 1458 1099

Density 0.829

Flow [% of TB] 43 57 71 80 85 90 100

Flow [m3/sec] 261 349 431 486 519 541 610

Pressure [mmca] 656

Fan efficiency [%] 18 27 32 40 55 70 85.8

Fan power, damper [%] 48 61 73 81 85 90 100

Power [kW], (vanes) 1’825 2’305 2’753 3’053 3’233 3’397 3’791

VFD efficiency [%] 95 96 97 97 97 97 97

Power [kW], (VFD) 313 740 1’386 1’977 2’407 2’849 3’909 From the data in Table 3, the yearly energy consumption is calculated by multiplying the hours of

operation for each loadpoint with the corresponding power requirement. Figure 9 shows the result

of a comparison of energy consumption between a fixed speed and variable speed fan, when

electricity is valued at EUR 45 / MWh. It can be seen that the variable speed fan generates savings

in all the generator’s normal operating points, even at the nominal load. This is due to the fact that

the equipment is over-dimensioned, and the operation at part load with mechanical control

technologies is inefficient.

-0.150

-0.100

-0.050

0.000

0.050

0.100

0.150

0.200

0.250

0.300

48% 64% 79% 89% 95% 99%

Loadpoint

ME

UR

Damper VFD Savings, MEUR

Figure 9: Distribution of power consumptions of fixed speed and variable speed ID fan at the annual operation

points.

Using some typical equipment prices of a suitable VFD system, the following investment pay-back chart can be

created,

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Figure 10:

I.D. Fan, Electricity price €45/MWh, interest 6%

-0.16-0.12-0.08-0.040.000.040.080.120.160.200.240.28

0 1 2 3 4 5

Years after installation

ME

UR

/ ins

talle

d Fa

n M

W

NPV of savings, cumulative

Figure 10: Annual energy saving benefits compared with initial net investment of a variable speed drive system

The figures in the year of installation can be divided in two parts, depending on the actual situation;

the “net VFD investment” is the additional necessary investment of a VFD system, while the “NPV

savings” in the year “zero” means the alternative investment, like a new, larger fixed speed motor.

In the latter case the fan would be upgraded and a motor with bigger capacity would be required. It

must be noted that the above calculation is applicable for one of the two installed fans.

In this illustrated case, it can be concluded that the pay-back period of a VFD system for controlling

an ID fan becomes less than 2 years and this without considering any reduction in maintenance

costs. Such additional savings can be treated similarly to the savings in the annual energy costs.

In the above case the steam generator was operated most of the time between 72 and 92 % of the

660 MW generator’s output (refer to Figure 2). In Table 3 can be seen that the power consumption

is 1 – 1.4 MW less in each of the two ID fans, which represents almost 0.4% of the generator’s

nominal output.

Similarly, at the 79% load (521MW), the ‘saved’ 2 x 1.4 MW means actually more than 0.5%

improved plant efficiency. The additional 2 x 1.4 MW also contribute to the production increase

through an increased number of operating hours. The plant becomes more competitive at partial

load.

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The variable speed fan control can be considered as a simple and inexpensive way to increase the

power of the power plant. There are no additional operation or fuel costs and no CO2 emissions. It

has a positive effect on the plant controllability and due to the smooth starting capability it reduces

mechanical maintenance and extends equipment lifetime.

7. Advantages of variable speed fans driven by medium voltage AC drives

Variable speed fans driven by medium voltage AC drives have the following advantages over

conventional inlet vane or inlet damper controlled fans:

7.1. Energy efficiency

The outstanding efficiency of variable speed fans has been discussed in this paper. The fan input

power varies as the cube of the speed and while satisfying the system requirements at maximum

continuous rating or at lower loads, power savings are maximized as compared to any other method

of flow control. Sometimes it is difficult to match all three parameters (flow, pressure and speed) at

the maximum efficiency point. To select a fan at maximum efficiency, sometimes the fan needs to

be selected at a speed other than synchronous speed, which becomes possible with a variable speed

drive. Also, a fan has higher efficiency when inlet vanes or inlet dampers are totally eliminated.

Through the energy efficiency of variable speed fan, investment in electric variable speed drives

can typically prove to be the most economical choice in all cases with longer than few years

operating period. This is especially the situation in cases where investment must be split over

several years and the alternative is to install and operate a heavily throttled fixed speed fan

dimensioned to future demand.

7.2. Reliability, controllability, operability

Faster and better control together with an increased control range are major reasons for considering

variable speed drive control.

Controllability – This is a major advantage over any damper or vane controlled fans. Flow varies

directly proportional to speed and pressure varies with the square of the speed. Additionally, the

system resistance (pressure drop) varies with the square of the flow. The superior control of these

parameters, flow and pressure, is achieved by adjusting the speed of the fan with an electric variable

speed drive. With inlet vanes or inlet dampers the control is not as precise.

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The reliability of the system is greatly increased because there are no other moving parts in the

system.

Power-Loss-Ride-Through – Some medium voltage AC drives have the ability to ride-through

prolonged voltage fluctuations (e.g. transient voltage dips) on the supply network without tripping.

This feature is an advantage compared to DOL (Direct-On-Line) motors, which will trip in a shorter

period of time. This feature, together with “Flying Start” (when a drive can catch a spinning fan –

for example after an extended loss of supply) results in a fan drive system, which is robust towards

fluctuations in the supply network, refer to Table 2.

Lower noise – Due to the fact that dampers are eliminated in the fan for flow control and the

elimination of throttling action, the noise level is considerably lower. This usually amounts to 8 to

10 dBA noise reduction.

Furnace implosion may be a concern in power plants, meaning the ID fan could cause the boiler to

collapse due to too much under-pressure in the furnace. The implosion may occur as a result of a

control malfunction or through operator error, for instance the dampers on the FD fan are closed

while the ID fan is in operation.

Risk of furnace implosion can be reduced with variable speed fan control, because of the following

known operational characteristics.

First, as learned, a variable speed fan operating at MCR has considerably lower speed than the

design speed (TB) of the corresponding fixed speed fan. This means lower pressure, and lower risk

of implosion, at zero flow.

Secondly, a variable speed fan can be controlled to reduce speed immediately upon detection of a

dangerous situation. The operation point will thus route towards zero head of the head capacity

curve, unlike the situation of a fixed speed fan, which has the characteristic of increased pressure

with declining flow. Further, an electrical variable speed control will be actively controlled during

the ‘emergency’ speed reduction. A fixed speed fan motor will slowly coast down towards zero

speed when the circuit breaker is opened, and depending on the system design, it may not be

possible to re-start the motor directly across the line, as long as speed of the rotating inertia has

decreased under a defined minimum starting speed.

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7.3. Soft starting capability

There is no motor inrush current (which typically can be 650% of the full load current), which

means less wear and tear and increased lifetime of the motor, coupling and fan. Additionally, the

impact on the supply network is greatly reduced, meaning less likelihood of causing problematic

voltage drops during starting. The inherent feature of the variable speed drive makes it possible to

nullify the effects of inertia and the rate of acceleration of the system can be adjusted or

programmed.

7.4. Reduced maintenance

Inlet vane or damper control usually requires some periodic maintenance and elimination of inlet

vane controls reduces maintenance costs.

As the speed is reduced, the peripheral velocity of the fan rotor is reduced and, as a result,

impingement and sliding velocity of the particles in the gas is greatly reduced compared to constant

speed fans. This increases the fan lifetime and reduces maintenance.

Large ID fans equipped with inlet dampers at certain damper settings can cause a rotating stall. This

can result in ductwork vibrations of very high magnitude, which forces the user not to operate the

fan at the onset of stall or below. Inlet vane controls can cause cone vortex problems because of too

much swirl and can cause ductwork pulsations in the discharge side ductwork. Speed control of the

fan eliminates the use of damper or vane control and, thus, the pulsation problems are avoided [2].

7.5. Reduced investment in other parts of plant

Because of the soft start capabilities, a less expensive motor having lower inertia capabilities can be

utilized (as long as there is no drive by-pass requirement). When the motor power can be reduced,

savings can also be achieved in the electrical distribution system, as voltage drop due to direct on

line starting is no longer an issue. The distribution system can have a lower fault level. .

Variable frequency drives offer the possibility to rotate the fan at a lower speed to avoid thermal

stratification, thereby eliminating the need for a turning gear.

Ability to start a hot gas fan at ambient conditions at slow speed – Though power consumption

varies directly with the density of gas, with speed control, power consumption varies as cube of the

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speed. As such, a variable speed fan makes it possible to operate a hot gas fan at ambient conditions

at lower speed, and eliminates the need for a two-speed motor or a larger motor.

8. Medium voltage AC drives

Medium voltage AC drives

Medium voltage AC drives are available in a variety of arrangements, depending on whether the

motor to be controlled is asynchronous or synchronous. For example, asynchronous motors (such as

cage induction motors) are typically controlled by Voltage Source Inverters (VSIs). Synchronous

motors for fan applications are typically controlled by Load Commutated Inverters (LCI).

Voltage source inverter

Today, state-of-the-art medium voltage AC drives (as shown in Figure 11) combine the VSI

technology with efficient power semiconductors, excellent motor control software and a sine wave

output. These technologies combined with a standard cage induction motor form an ideal electric

drive system with the following advantages:

• Operates on a standard, robust, low maintenance cage induction motor

• Retrofits onto existing motors possible. Fixed speed fans can be converted to variable speed

(after having checked with the fan manufacturer)

• High drive system efficiency

• Low network harmonics

• High power factor (close to unity)

• Elimination of variable speed drive induced torque pulsations

• Small footprint (an advantage for retrofits)

• Quieter motor operation

22

3

IsolationTransformer

Rectifier DC-Link

Inverter Filter Motor

NInd.Motor

di/dt-choke

Figure 11: MV AC drive using the VSI topology, IGCT power semiconductors and sine wave output filter

9. Summary

• Operating point effects fan efficiency

• Fans are usually over dimensioned

• There is always need to control

• Summary of the main benefits of large variable speed ID fans driven by medium voltage

AC drives:

Feature: Benefit:

High efficiency • Reduced energy costs

Soft starting • Reduced stress on fan, motor and coupling, resulting in extended equipment lifetime

• Reduced maintenance costs

• Reduced impact on the electrical supply network (no voltage drop due to starting)

• Elimination of problems with starting high inertia loads

• Programmable acceleration ramps and profiles

• Unlimited number of starts per hour

• Reduced cost. (motors do not need to

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be sized for high inertia capability

High reliability and availability • Reduce downtime

• Improved process availability

Lower audible noise • Improved working environment

Low fan erosion at lower speeds • Extended fan lifetime

• Reduced maintenance costs

Flexible motor speed • A fan with other speed than defined by motor at 50 or 60 Hz can be selected

• Flexibility in operation

Elimination of pressure pulsations • Reduced ductwork vibration and fatigue

• Lower noise

Elimination of inlet vanes and dampers • Reduced maintenance costs

Ability to rotate fan at lower speed • Eliminates the need for a turning gear

Ability to start a hot gas fan at ambient

conditions at low speed

• Eliminates the need for a two-speed motor or a larger motor

Precise flow control • Improved controllability

• Improved process stability

Wide speed control range • Flexibility in operation

Power-Loss-Ride-Through • Increased process availability due to reduced number of trips

About ABB MV Drives: ABB’s research into AC drives technology started in the 1960s. This long

history in the AC drives business has brought the company vast experience in AC drives technology

as well as application know-how in a wide range of industries, including the Utility Industry. ABB

is the world’s largest drives manufacturer, and is the world leader in medium voltage AC drives.

About ABB: ABB has ISO9001 certified manufacturing facilities, stringent quality inspection

requirements and an integrated project management team approach to ensure quality products

delivered on time to meet client expectations.

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References:

[1] IEEE Std 958-2003 - IEEE GUIDE FOR THE APPLICATION OF AC ADJUSTABLE-

SPEED DRIVES

[2] Varanasi, et al. : SCR Retrofits Provide Opportunity For Variable Speed Fans, Power

Engineering November, 2000

[3] Hiroaki Yamada et. Al.: High-voltage Direct Inverter Applied to Induced Draft Fan Motor at

Takehara Thermal Power Station No. 3 of Electric Power Development Co., Ltd. 2004.

[4] Nathan Schachter, CIMENTEC Engineering Ltd: Energy Efficient Speed Control Using

Modern Variable Frequency Drives

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